Samples:
• Unbiased (central tendency)
• Stable (precision/variability)
Stability
More
Ideal: a stable unbiased sample
PoP
PoP
PoP
PoP
PoP
PoP
PoP
PoP
PoP
PoP
More
Bias
Less
Less
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Like playing darts…
Always aim for the center…
Stable
Stable
And not biased...
But… biased
From samples to population…
Simple random sampling
homogeneous population
Ex. women only
Stratification in sampling
known composition
of the population
Unstable
But… not biased
60% women; 40% men
Stratum
(cluster)
Stratum
(cluster)
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Point estimate
Estimating the population
Survey:
100 students out of 2500 graduating seniors
Sample
25: YES to graduate school -->
25% of 2500= 625
Interval estimate
25% ± 9% --> interval of 95% of true population
400 to 850 (confidence interval)
Bias due to nonresponse
in survey research
Estimate of
Population
values
Sample
values
Error of
Estimate
Team A & B
11.5, 12.5
12.00
-2.00
Team A & C
11.5, 13.0
12.25
-1.75
Team A & D
11.5, 19.0
15.25
+1.25
Team B & C
12.5, 13.0
12.75
-1.25
Team B & D
Team C & D
12.5, 19.0
13.0, 19.0
15.75
16.00
+1.75
+2.00
Mean: 14.00
0.00
Characteristics of volunteer participants:
• > Education
• > Social status
• > Intelligence “Open-mindness”
• > Social approval
• > Sociability
• High sensation (men)
• More women
• < Conventional
• < Authoritarian
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